If you want to be using it this year, buy a complete van. A few considerations:
Convertors will probably know what to look for in a base van and with experience, will get them cheaper than you can.
The VW warranty isn't necessarily all that useful is it? What are you worried about going wrong?
Conversions are often designed, it seems, for two adults rather than young kids - so having lots of furniture means no space for pushchairs, bikes etc. A pop top makes a very useful 'play loft' in poor weather - arrive at campsite, pop top and throw lego/books/tablet and kids in there while you get sorted. They often seem to have complex switch panels and Electrical Consumer Unit type stuff, which is unnecessary.
If the kids are very young, a Kombi with passenger seats near the driver is really useful and there are various bed and pod system that then use the Kombi seats. Leaves a family bus with loads of boot space for normal life. We've had a Kombi (T5 now T6) for 10 years and I'm only now adding some fixed furniture and a kitchen.
With current lead times on parts and conversion work, you'd do well to have one converted before the Christmas lights go up.
It is satisfying and fun to be speccing your own and perhaps DIYing parts of the work.
I'd be looking for both options and see what comes along.